In recent years, gene therapy is a medical treatment which can be expected to rehabilitate and normalize, at a gene level, irregularities causing various diseases. However, the gene is very unstable. For example, in the case where the gene is administered as-is, the gene undergoes decomposition due to nuclease and the like in vivo. Therefore, selection of a vector used for gene therapy is very important.
Virus vectors used in the past are effective vectors exhibiting a high degree of gene transfer efficiency. However, cases in which clinical trials linked to death have also been reported, so that there is anxiety for the safety. Consequently, development of nonviral vectors with high safety and high gene introduction efficiency has become the mainstream and research has been performed actively all across the world.
Nonviral vectors include complexes of cationic polymers, e.g., polyethylene imine, and genes (polyplex) and complexes of liposomes made from cationic lipids and genes (lipoplex).
Most of all, the lipoplex is a vector most generally used in the present because of being functionalized easily in such a way as to have improved retentivity in blood, accumulate to a target cell, and the like depending on the type of lipid mixed and is expected greatly to go into actual use.
As for a typical example of the lipoplex, Wheeler et al. have reported that a stabilized plasmid lipid particle (SPLP) containing dioleoylphosphatidylethanolamine (DOPE) serving as a fusogenic lipid, a cationic lipid, or the like is coated with a polyethylene glycol lipid (NPL 1). In the report, as for the cationic lipid, in order to improve a drug delivery efficiency, the lipid having fusogenic properties and containing one unsaturated bond, e.g., an oleyl group, (for example, DODAC (dioleyldimethylammonium chloride)) is used.
Moreover, there is a report in which the drug delivery efficiency is further improved than ever by using a cationic lipid having at least two unsaturated bonds, e.g., a linoleyl group, and exhibiting enhanced flexibility (PTL 1). In addition, there is a report in which an analogous cationic lipid is used (PTL 2).
Regarding the cationic lipids used therein, a lipid portion and a skeleton having a cationic group are bonded with an ether group. Consequently, significant stability is exhibited because hydrolysis is not effected in contrast to the cationic lipid and the like in which a lipid portion and a skeleton having a cationic group are bonded with an ester group.
In general, in the case where such a cationic lipid including the bond with the ether group is synthesized, the method of Williamson et al. is used in which a compound having a cationic group and a hydroxyl group and a lipid compound having a leaving group are reacted in an organic solvent in the presence of a strong base at high temperatures. Examples of concrete methods for manufacturing a cationic lipid compound include production from 3-dimethylamino-1,2-propane diol containing a cationic group (dimethylamino group) and a long chain fatty bromide or a long chain fatty methane sulfonate in an aromatic solvent, e.g., benzene, toluene, or xylene, in the presence of a strong alkali catalyst, e.g., sodium hydroxide or potassium hydroxide at high temperatures, e.g., reflux (PTLs 1 and 2).
In general, it is known that a fatty acid, e.g., linoleic acid, having a methylene group sandwiched between two cis form double bonds in the molecule is oxidized easily and is isomerized easily under the conditions of high temperatures, alkali, and the like because the methylene group has high reactivity. The isomerized fatty acid (trans fatty acid) exhibits a different bioactivity. There is a report that, for example, excess intake increases the risk of decease. Therefore, an adverse effect may be exerted in the body.
Meanwhile, it is known that a fatty acid, e.g., oleic acid, having one double bond also undergoes oxidation because methylene is included on both sides of the double bond, although the reactivity is especially lower than the reactivity of the methylene group sandwiched between adjacent two cis form double bonds in the molecule of linoleic acid or the like and, therefore, isomerization does not occur easily.